1. Field of the Invention
The present invention relates to an operational amplifier, especially to one to be used in a device for driving a liquid crystal panel.
2. Description of the Prior Art
Typically, a liquid crystal panel requires a writing operation at a speed of several tens of frames (several tens of sheets) per second. An output signal generated from a drive circuit of the liquid crystal panel is provided for performing an AC drive on the potential of a common electrode for each of scanning lines or frames. Referring now to FIGS. 7 and 8, we will describe an operational amplifier and a drive circuit of the conventional liquid panel, which perform such an AC drive on the potential of common electrode.
FIG. 7 is a circuit diagram that illustrates an example of the conventional operational amplifier for driving a liquid panel. As shown in the figure, the conventional operational amplifier 1a comprises differential input stage circuits 2, 3, output stage field effect transistors (FETS) 11-14, and drive stage circuits 4, 5, and it may be functionally used as a converting circuit for an output impedance. In the operational amplifier 1a, each of the differential input circuits 1, 3 is connected between a high-potential side power source (VDD) 8 and a low-potential side power source (VSS) 9. The differential input circuit 1(3) amplifies the differential potential between an analog input supplied to a positive input terminal 11(13) and an analog input supplied to a negative input terminal 12(14) and generates an output to a differential input stage output terminal 101 (102). One end of the FET 11(13) is connected to the high-potential side power source 8 and the other end thereof is connected to an output terminal VO1(VO2) of the operational amplifier 1a. In addition, one end of the FET 12(14) is connected to the low-potential side power source (VSS) 9 and the other end thereof is connected to an output terminal VO1(VO2) of the operational amplifier 1a. Each of the drive stage circuits 4, 5 is also connected between the high-potential side power source 8 and the lower-potential-power source 9. The drive stage circuit 4(5) supplies a drive output signal to the FETs 11 and 12 (13 and 14) through output terminals 105 and 106 (107 and 108) on the basis of the differential outputs from the output terminals 101, 102, respectively.
Each of the differential input stage circuits 2, 3 of the operational amplifier 1a is able to acquire the input range from a level at the low-potential side power source (VSS) to a level at the high-potential side power source (VDD). The output stage FET 11 has a gate electrode connected to an output terminal 105 of the drive stage circuit 4, a source electrode connected to a high-potential side power source 8, and a drain electrode connected to the output terminal VO1. Similarly, the output stage FET 13 has the connections to the drive stage circuit 5 and the output terminal VO2. Similarly, the output stage FETs 12, 14 have their connections to the low-potential side power source 9 and the output terminal VO2.
FIG. 8 is a circuit diagram that illustrates the configuration of an example of the circuit for driving the liquid crystal panel (hereinafter, simply referred to as a LCP-drive circuit) in which the conventional operational amplifier is used. As shown in the figure, the LCP-drive circuit 40a comprises: positive and negative side digital-to-analog (DA) converters 41, 42 that translate digital signals to analog signals with respect to input signals on the positive and negative sides, respectively; switching means 43, 44 for switching the translated outputs from the DA converters using the predetermined input control signals from the outside; the operational amplifier (see FIG. 7) for the operationally amplifying the outputs switched by the switching means 43, 44; and switching means 47, 48 for switching the outputs VO1, VO2 from the operational amplifier using control inputs from the outside and then supplying the outputs to output terminals OUT1, OUT2, respectively.
The DA converters 41, 42 perform digital to analog transformation to obtain analog data of middle-potential to high-potential side power source and analog data of middle-potential to low-potential side power source, respectively, depending of input digital data. Each of the switching means 43, 44, 47, 48 is constructed of a pair of switches S and Sb that perform different operations opposed to each other. Furthermore, the operational amplifier creates the negative feedback of signals, so that each of the outputs VO1, VO2 is feed backed to negative side inputs VI2, VI4 against positive side inputs VI1, VI3, respectively.
The LCP-drive circuit 40a can be actuated and operated as follows. At first, analog signals from the positive side DA converter 51 and analog signals from the negative side DA converter 42 are respectively introduced into the operational amplifier 1a when each switch S in the switching means 43, 44, 47, 48 is switched on (at this time, the switch Sb is switched off). Then, each input signal is subjected to an impedance conversion and is then generated as an output to the output terminal OUT1 or OUT2 through the switching means 47 or 48. In general, a plurality of output terminals is provided on the LCP-drive circuit 40a for driving each element of the liquid crystal panel. For simplifying the illustration and for the sake of expediency, the circuit 40a is described as one having two output terminals.
Likewise, when each switch Sb in the switching means 43, 44, 47, 48 is switched on (at this time, the switch S is switched off), analog signals selected with the positive side DA converter 41 is subjected to an impedance conversion and is then generated as an output to the output terminal OUT2, while those selected with the negative side DA converter 42 is subjected to an impedance conversion and is then generated as an output to the output terminal OUT1.
The LCP-drive circuit 40a is able to generate several tens of outputs of positive- or negative-side analog signals (i.e., to perform several tens of writing operations on the panel). If the scanning line is switched from one to another, then the terminal from which the negative side analog signals are outputted and the terminal from which the positive side analog signals are outputted are replaced with each other to operate with alternating current.
FIG. 9 is a timing chart of an output waveform of the conventional LCP-drive circuit. As shown in this figure, if the opposite switching operations of switches S, Sb are performed, signal waveforms for the discharge of the liquid crystal panel to be outputted to the output terminals OUT1, OUT2 may be changed from the voltage at the high-potential side power source VDD to the voltage at the low-potential side power source VSS and from the voltage at the low-potential side power source VSS to the voltage at the high-potential side power source VDD, respectively.
The liquid crystal panel described above is provided as a capacitive load. As for driving such a liquid crystal panel due to the change in analog signals to be inputted, therefore, it means that the capacitive load of the panel can be charged and discharged.
As described above, furthermore, the LCP-drive circuit repeats the operation in which the positive- or negative-side voltage is outputted several ten times, the output polarity is then replaced, and the negative- or positive-side voltage is subsequently outputted several ten times.
The charge and discharge of the capacitive load is performed between the high-potential side power source and the low-potential side power source, so that a power consumption P per one output can be expressed by the following equation.
P=Cxc3x97fxc3x97Vppxc3x97VDD
Wherein VDD denotes the potential difference between the high-potential side power source and the low-potential side power source, Vpp denotes a writing amplitude, f(Hz) denotes a writing frequency, and C denotes the capacity value of the capacitive load of liquid crystal panel.
Therefore, the conventional operation amplifier and the conventional LCP-drive circuit using such an amplifier described above has a problem that the power consumption P can be increased as the potential difference between the high-potential side power source and the low-potential side power source can be VDD (when VSS equals to zero volt) even though the writing of the positive- or negative-output voltage is only performed several ten times.
Furthermore, when the liquid crystal panel is operated with alternating current, the liquid panel should be designed to reduce an unevenness of its display to a minimum.
Object of the Invention
An object of the present invention is to provide an operational amplifier that is able to reduce an unevenness of display on the liquid crystal panel to be generated at the time of operating the liquid crystal panel with alternating current in addition to decrease the amount of charge or discharge power of the panel load to be consumed at the time of operating the liquid crystal panel with alternating current.
Summary of the Invention
In accordance of the present invention, there is provided an operational amplifier that comprises: a first differential input stage circuit having a differential input terminal including a first positive input terminal and a first negative input terminal and a first output end, which is connected between a low-potential side power source and a high-potential power side to ensure an input range from a level at the low-potential side power source to a level at the high-potential side power source; a second differential input stage circuit having a differential input terminal including a second positive input terminal and a second negative input terminal and a first output end, which is connected between a low-potential side power source and a high-potential power side to ensure an input range from a level at the low-potential side power source to a level at the high-potential side power source; a first drive stage circuit having a first input end, a third output end, and a fourth output end, which is connected between the low potential side power source and the high potential side power source; a second drive stage circuit having a second input end, a fifth output end, and a sixth output end, which is connected between the low potential side power source and the high potential side power source; a first semiconductor device in which a first electrode is connected to a third output end of the first drive stage circuit and a second electrode is connected to the high-potential side power source, and a third electrode is connected to the first output terminal; a second semiconductor device in which a first electrode is connected to a fourth output end of the first drive stage circuit and a second electrode is connected to a middle-potential side power source, and a third electrode is connected to the first output terminal; a third semiconductor device in which a first electrode is connected to a fifth output end of the second drive stage circuit and a second electrode is connected to the middle-potential side power source, and a third electrode is connected to the second output terminal; a fourth semiconductor device in which a first electrode is connected to a sixth output end of the second drive stage circuit and a second electrode is connected to the low-potential potential side power source, and a third electrode is connected to the second output terminal; a first switching means having switches respectively connecting to the first output end of the first differential input stage circuit and the first input end of the first drive stage circuit and the second input end of the second drive stage circuit, in which the switches are operated in reverse with each other; and a second switching means having switches respectively connecting to the second output end of the second differential input stage circuit and the first input end of the first drive stage circuit and the second input end of the second drive stage circuit, in which the switches are operated in reverse with each other.